You would have to be way past a certain age to remember the concert scene in this country before Bernard Haitink began appearing here. The great Dutch conductor has been around for so long and has been so prominent - first as head of the London Philharmonic, then in charge of Glyndebourne and Covent Garden, in addition to his regular guest work with a range of orchestras - that it is hard not to think that he must somehow be immortal.

Even so, it still comes as something of a shock to realise that Haitink celebrated his 75th birthday this week. His hair - not that there has been much of that for many years - is white now, and if you meet him after a concert, as I did in Berlin recently, the age and the tiredness can show. Yet when Haitink makes his entrance on the orchestra platform, the years fall away. The way he holds himself and the way he makes music - attention-compelling but never attention-seeking - these things have not changed since he first emerged in the 1960s.

"I have to be aware of my age," Haitink says. "But if I were to feel like an old man, I would not be able to do it any more, because conducting needs a lot of physical and mental strength, and I am under the illusion that I can still offer that to musicians."

Outside of the leadership of the Chinese Communist party, there can be no profession in which 75-year-olds are considered relative striplings. Toscanini and Klemperer were still conducting in their 80s, and Stokowski was giving concerts in his 90s.

"We conductors have one huge advantage," Haitink points out. "We are not instrumentalists. At 75, it's hard to sing any more - your voice goes; or to play any more - your fingers are rheumatic. But the technique of conducting is not as subtle as that. It demands other things that apparently you can still do when you are older.

"I think maybe conducting is not something for young people. I started far too young with a world-famous orchestra. I still have sleepless nights about it sometimes - how was it possible that I could do this and that without any musical or human experience? It is a miracle that I survived."

Over the next few months, he will be marking his 75th with a series of concerts at the Barbican in London in which he will appear with five of the most important orchestras of his career - and hence in European music. The series starts later this month with the Concertgebouw of Amsterdam, of which he became principal conductor 40 years ago, and continues with the Vienna and Berlin Philharmonics, the London Symphony and finally - in November - the only orchestra of which he is now music director, the Dresden Staatskapelle.

How does he characterise these mighty beasts of the orchestral jungle? Haitink starts his reply with the Berliners, with whom he is doing a two-week stint as we speak. "I love the open way they attack the music. It is so positive. When they play with conductors they don't like, they ignore him; but when they play with conductors they like, they really add something very positive."

What about Vienna? "Well, you never know with Vienna because they have an enormous number of players, and you have to wait and see who plays. They don't have a music director. They play the same pieces more than once in a season with different conductors. I think in their hearts they are arrogant. They think, 'It doesn't matter who conducts us, we are the Vienna Philharmonic.' Very dangerous attitude. But they are of course extremely good musicians."

Next, the Concertgebouw, of which he remains honorary conductor: "Totally different. It is a young orchestra nowadays, like the Berlin, and with younger people comes a technical ability that is amazing.

"The Dresden Staatskapelle is in between. It's in between Vienna and Berlin too. And I'm very fond of that Dresden orchestra. I am sad that it only came in my life so late, because I would have loved to have a longer spell with them."

What about London? "That's really a difficult situation," he reflects, "because London has a pool of musicians that is unique. Boulez says you can do what you want in London, because for every piece there are always musicians who can play it. The tragedy is the way orchestras have to operate. Concert life just survives by the skin of its teeth. The whole life in London, I don't have to tell you, is so difficult. Nearly inhuman. Not much pay. Terrible pressure. Managers of orchestras need all the time to raise the money and can't concentrate on musical standards. The players are incredible. It's the survival of the fittest.

"It is tragic that London doesn't have a really good hall, and that it doesn't just have two well-established orchestras that don't have to fight for their lives. But I think it will never change, that's the sad thing. The political agenda is not interested. Nevertheless one can do extraordinary things."

Does that mean that Haitink with the LSO is completely different from Haitink with the Berlin Phil? He thinks not. "Every good conductor brings his own sound to any orchestra. It's a matter of getting something out of them with your authority. It's difficult to put your finger on it, but I go for the same sound with every orchestra I conduct - transparency, warm sound in the strings but not at the cost of transparency, well-balanced brass - that's the grammar of all my conducting."

How does he get it to work? "Difficult question. But you find out if it does after five minutes, that's all I can say. I don't have any recipe. I know there are conductors who have special tricks to get the attention of the orchestra, but I don't like that. The first rehearsal is always, well, not nerve-racking, but you are always on tiptoe.

"So with the Berliners and the Brahms Fourth that we played last night. Remarkably, they haven't played it recently, which is very lucky for me. In this particular Brahms, I spotted that they play it very well, but too loudly and not enough dynamic differences, so I started to work on that. Not with many words; when you start to talk to orchestras then you are losing it."

He talks about conductors he admires. The list starts with Carlos Kleiber. Simon Rattle tells the story of how he and Haitink were sitting in a Covent Garden box at a closed Kleiber rehearsal of Otello. When it ended, Haitink turned to Rattle and said: "Well, I don't know about you, but I think that my studies in this art have only just begun."

"Yes, that is true," he says now. "I'm not ashamed of saying that. When I have listened to Kleiber, I always think, 'My God, he knows his scores so well.' He is a fanatic. He looks at every manuscript and he will dig out every note, every detail, every query." For the young Haitink, though, the key figure was Wilhelm Furtwängler. "I remember specially in Salzburg, in 1948 I think, when I heard Fidelio there and Bruckner's Eighth. And I also heard the same week the young Karajan - that was not so good. "I went to Fidelio with tremendous expectations. I was trembling with excitement." He remembers seeing "this strange-looking man" coming into the pit and starting the overture. The horns, says Haitink, "made dominoes" to Furtwängler's notoriously imprecise downbeat. "And I thought, 'Well, is that the great Furtwängler?'

"Then, all of a sudden, something happened and it was as if there was electricity in the auditorium, and that stayed and it built itself up more and more, and at the end ... the next morning, there was the concert of the Bruckner Eighth and again I had this fantastic experience, I was totally emotional about it. Strangely enough, when I heard a recording of Furtwängler's Bruckner Eighth I hated it."

No three composers have loomed larger in Haitink's lifetime on the podium than Bruckner, Mahler and Shostakovich, and all have pride of place in the year's celebrations. Shostakovich's Eighth Symphony is the main work in Haitink's March 20 concert with the Concertgebouw. He calls it "an Amsterdam piece" with "a special place in our hearts". The following night, he conducts Bruckner's Ninth. "You must believe in Bruckner," he says. "Don't mess around with it. Don't try to impose yourself, because that is counterproductive. You have to deliver yourself to this endless shape, especially in the finales. It's a journey and you have to be the guide." Do you have to be a Christian to believe in Bruckner? "I am not," Haitink responds crisply.

Then, inescapably, there is Mahler. These days, when everyone performs Mahler at the drop of a hat, there will be many concert-goers who do not realise what a Haitink Mahler evening is like, or how significant he was in opening the world's ears to a composer who, until around 1960, was often still regarded as a curiosity. They should make a point of going to one of the three great Mahler symphonies that are at the core of the Haitink celebrations - the Third (with the Berlin Philharmonic in September), the Sixth (with the LSO in June) and the Ninth (with the Vienna Philharmonic in April).

Haitink especially looks forward to the Third: "The Berlin Philharmonic has played a lot of Mahler, but Simon has said to soft-pedal it. But he made an exception for me because I said I would like to do Mahler Three. It's a huge canvas, with wonderful things in it, and difficult to organise. It is not a routine thing. I said I would like to do it once more in my life."

He will go on as long as he can in the concert hall, but has decided he will never conduct again in the opera house. "No, that's it. Covent Garden asked me to do Meistersinger again in 2007, but it would be idiocy, even if I'm still around. It's a great shame.

"I'm quite philosophical about getting older. I don't want to plan on the basis of saying this is the last time I can do things. I don't like that. In the worst of all cases, I maybe put a question mark against an idea. But what can you do? I'm 75. I can't ignore that."

· Haitink at 75 is at the Barbican, London EC2, from March 20. Box office: 0845 120 7550.